Liquid crystal (LC) materials are rod-like or lath-like molecules which have different optical properties along their long and short axes. The molecules exhibit some long range order so that locally they tend to adopt similar orientations to their neighbours. The local orientation of the long axes of the molecules is referred to as the “director”. There are three types of LC materials: nematic, cholesteric (chiral nematic), and smectic. For a liquid crystal to be used in a display device, it must typically be made to align in a defined manner in the “off” state and in a different defined manner in the “on” state, so that the display has different optical properties in each state. Two principal alignments are homeotropic (where the director is substantially perpendicular to the plane of the cell walls) and planar (where the director is inclined substantially parallel to the plane of the cell walls). In practice, planar alignments may be tilted with respect to the plane of a cell wall, and this tilt can be useful in aiding switching. The present invention is concerned with alignment in liquid crystal displays.
Hybrid Aligned Nematic (HAN), Vertical Aligned Nematic (VAN), Twisted nematic (TN) and super-twisted nematic (STN) cells are widely used as display devices in consumer and other products. The cells comprise a pair of opposed, spaced-apart cell walls with nematic liquid crystal material between them. The walls have transparent electrode patterns that define pixels between them.
In TN and STN displays, the inner surface of each wall is treated to produce a planar unidirectional alignment of the nematic director, with the alignment directions being at 90° to each other. This arrangement causes the nematic director to describe a quarter helix within the TN cell, so that polarised light is guided through 90° when a pixel is in the “field off” state. In an STN cell, the nematic liquid crystal is doped with a chiral additive to produce a helix of shorter pitch which rotates the plane of polarisation in the “field off” state. The “field off” state may be either white or black, depending on whether the cell is viewed through crossed or parallel polarisers. Applying a voltage across a pixel causes the nematic director to align normal to the walls in a homeotropic orientation, so that the plane of polarised light is not rotated in the “field on” state.
In a HAN cell, one wall is treated to align a nematic LC in a homeotropic alignment and the other wall is treated to induce a planar alignment, typically with some tilt to facilitate switching. The LC has positive dielectric anisotropy, and application of an electric field causes the LC directors to align normal to the walls so that the cell switches from a birefringent “field off” state to a non-birefringent “field on” state.
In the VAN mode, a nematic LC of negative dielectric anisotropy is homeotropically aligned in the “field off” state, and becomes birefringent in the “field on” state.
Liquid crystal (LC) planar alignment is typically effected by the unidirectional rubbing of a thin polyimide alignment layer on the interior of the LC cell, which gives rise to a unidirectional alignment with a small pretilt angle. It has been proposed to increase the pretilt angle for a rubbed surface by incorporating small projections in the rubbed alignment layer, in “Pretilt angle control of liquid-crystal alignment by using projections on substrate surfaces for dual-domain TN-LCD” T. Yamamoto et al, J. SID, 4/2, 1996.
Whilst having a desirable effect on the optical characteristics of the device, the rubbing process is not ideal as this requires many process steps, and high tolerance control of the rubbing parameters is needed to give uniform display substrates. Moreover, rubbing may cause static and mechanical damage of active matrix elements which sit under the alignment layer. Rubbing also produces dust, which is detrimental to display manufacture.
Photoalignment techniques have recently been introduced whereby exposure of certain polymer coating to polarised UV light can induce planar alignment. This avoids some of the problems with rubbing, but the coatings are sensitive to LC materials, and typically produce only low pre-tilt angles.
An alternative is to use patterned oblique evaporation of silicon oxide (SiO) to form the alignment layer. This also effects a desired optical response; however the process is complicated by the addition of vacuum deposition and a lithography process. Moreover, control of process parameters for SiO evaporation is critical to give uniformity, which is typically difficult to achieve over large areas.
A useful summary of methods of aligning liquid crystals is given in “Alignment of Nematic Liquid Crystals and Their Mixtures”, J. Cognard, Mol. Cryst. Liq. Cryst. 1-78 (1982) Supplement 1.
The use of surface microstructures to align LCs has been known for many years, for example as described in “The Alignment of Liquid Crystals by Grooved Surfaces” D. W. Berriman, Mol. Cryst. Liq. Cryst. 23 215-231 1973.
It is believed that the mechanism of planar alignment involves the LC molecules aligning along the grooves to minimise distortion energy derived from deforming the LC material. Such grooves may be provided by a monograting formed in a photoresist or other suitable material.
It has been proposed in GB 2 286 467 to provide a sinusoidal bigrating on at least one cell wall, by exposing a photopolymer to an interference pattern of light generated by a laser. The bigrating permits the LC molecules to lie in two different planar angular directions, for example 45° or 90° apart. An asymmetric bigrating structure can cause tilt in one or both angular directions. Other examples of alignment by gratings are described in WO 96/24880, WO 97/14990 WO 99/34251, and “The liquid crystal alignment properties of photolithographic gratings”, J. Cheng and G. D. Boyd, Appl. Phys. Lett. 35(6) 15 Sep. 1979. In “Mechanically Bistable Liquid-Crystal Display Structures”, R. N. Thurston et al, IEEE trans. on Electron Devices, Vol. ED-27 No 11, November 1980, LC planar alignment by a periodic array of square structures is theorised.
LC homeotropic alignment is also a difficult process to control, typically using a chemical treatment of the surface, such as lecithin or a chrome complex. These chemical treatments may not be stable over time, and may not adhere very uniformly to the surface to be treated. Homeotropic alignment has been achieved by the use of special polyimide resins (Japan Synthetic Rubber Co). These polyimides need high temperature curing which may not be desirable for low glass transition plastic substrates. Inorganic oxide layers may induce homeotropic alignment if deposited at suitable angles. This requires vacuum processes which are subject to the problems discussed above in relation to planar alignment. Another possibility for producing homeotropic alignment is to use a low surface energy material such as PTFE. However, PTFE gives only weak control of alignment angle and may be difficult to process.
It is desirable to have a more controllable and manufacturable alignment for LC devices.